There have been many previous proposals for the treatment of aluminum metal sheets or plates to roughen and/or grain their surfaces so that they are more suitable in the manufacture of lithographic printing plates. See for example U.S. Pat. No. 4,477,317 and especially the disclosure in Column 1, which is incorporated herein by reference. As evident from the prior art, the electrolytic treatment has been one of the most popular methods for modifying the surfaces of the aluminum metal so that they can be effectively utilized as lithographic plate supports. Included among the prior art proposals have been various two-step electrolytic treatments.
In recent years the standards for commercially acceptable aluminum metal lithographic supports have become higher. More particularly, it has been found highly desirable to obtain an aluminum metal surface with a fine honeycomb structure that is substantially non-directional. Commercial considerations have also made it important to speed up the graining process operation so that more lithographic plates could be manufactured in a given period of time without incurring the costs that would necessarily result from significant changes in the processing operations or apparatus currently employed. It would be advantageous therefore to have a method for readily producing the fine honeycomb structure on the surface of the aluminum metal plates, while at the same time being able to speed up the production of the aluminum metal lithographic supports without encountering any untoward results.
Heretofore, a substantially non-directional honeycomb topography could be achieved by the method of U.S. Pat. No. 4,548,683 wherein the aluminum metal was immersed in an acidic bath comprising hydrochloric acid, nitric acid or mixtures thereof and utilizing higher frequencies of alternating current than conventionally employed for substantially long treatment times. One major disadvantage of this method is the need for special equipment to produce the requisite high frequencies. The use of nitric acid by itself in a single step, to obtain a proper average roughness of 0.5 to 0.7 .mu.m, has the further disadvantage of being too time consuming involving the use of large amounts of nitric acid, which in turn leads to major waste disposal problems thereby increasing the costs of such an operation. On the other hand, when hydrochloric acid is employed alone in a single step the results are untoward, since the resulting honeycomb structure is always undesirably directional, while at higher current densities no honeycomb was obtained though there was non-directional graining.